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Power Engineering II

Overvoltage Protection

Overvoltage Protection 2


Overvoltage Protection of Overhead Lines

• Usage of ground wires -> likelihood of ground wire striking is higher than the likelihood of phase conductors striking -> just prevention of the worst case, overvoltage can be still induced to the phase conductors

• Arrangement of ground wires on line is based on the theory of protecting zones

Z

Lα

hZhL

S

S

βS

Zone A Zone BZone A Zone C

Zone A – Lightning is likely hits the ground conductorZone B – Lightnint is likely hits the phase conductorProtecting angle α – determine the position of ground conductor above phase conductor

Protecting distance S – is given by empirical formula 𝑆 = 10𝐼2

3 [m; kA]



Overvoltage protection

• Fundamental classification– Spark (rod)-gaps

• Air spark-gaps with preset spark distance, connected between phase conductor and ground, permanent arc burning occures during the action-> failure state -> protection relays have to switch off the electric circuit

– Gapped surge arresters• Gap and nonlinear resistor (SiC material) in series, current is

limited to few amps after the overvoltage

– Metal oxide surge arresters• Gapless surge arresters, cylindrical blocks of nonlinear

resistors in parallel, Metal-oxide resistors (90% zinc oxide and 10% of various additives (Bi, Sb, Co, Mn)



Gapped surge arresters

• V-A characteristic and important parameters

Uzn

Ur

Umn

0 In i

u

In Peak value of rated discharge current

Umn vrcholová hodnota jmenovitého napětí

Uzn rated residual voltageUr rázové zapalovací napětí


Elektroenergetika 1

Gapped arresters (Valve type arresters)

• Construction • Both main gaps are ignited when the overvoltage occures at the terminals of arrester. At the same time, the current flows through the coil and nonlinear limiting resistor

• The current increase causes voltage across the coil , which iniciate ignition of auxiliary gap and bridging of coil

• The impadance of arrester is then given only as voltage drop across the nonlinear limiting resistor

• After the overvoltage decrease the impedance of coil decrease as well -> extinguishing of auxiliary gap and conection of coil into the circuit

• The magnetic field created by coil pushes the arc of main gaps to quenching chambers

Main gaps

Main gaps

Auxiliary gapCoil

Stabilizing resistor

Stabilizing resistor

Main gaps

Nonlinear resistor



Gapped Arresters

• Main parameters of gapped arresters– Rated ignition voltage at lightning impulse (1,2/50 µs)

• The lowest peak value of impulse which will causes the action of surge arrestor

– Residual voltage• Voltage drop caused by current impulse 8/20 µs

– Rated discharge current of arrester• The peak value of impulse 8/20 µs (1,5 kA, 2,5 kA, 5 kA, 10 kA, 20 kA,

40 kA)

– Rated voltage of gapped arrester• Highest RMS value of power frequency voltage across the arrester

terminals at which the arrester is closed

– AC ignition voltage of surge arrestor• RMS value of AC voltage at which the arrester opens



MO Surge Arresters

• Construction

Compression spring

Pressure relief diaphragm

Venting outlet Sealing ring

Cement joint

Metallic spacer

MO resistor

Supporting rod

Holding plate

Porcelain housing

Aluminum flange

Source: Volker Hinrichsen, Metal-Oxide Surge Arresters in High-Volgate Power Systems

Metal-Oxide (MO) resistors (ABB Switzerland Ltd.)

MO structure with grains and the boundaries between them (ABB Switzerland Ltd.)



MO Surge Arresters

• V-A characteristic

10-4 10-2 1 102 104

Peak value of continuous operating voltage: 2 𝑈𝑐 = 2 268 𝑘𝑉 = 379 𝑘𝑉

Peak value of phase-to-earth voltage: 2𝑈𝑠/ 3 = 2 242 𝑘𝑉 = 343 𝑘𝑉

Leakage current ≈ 100 µA Nominal discharge current In= 10 kA

Peak value of rated voltage: 2 𝑈𝑟 = 2 336 𝑘𝑉 = 475 𝑘𝑉

10-kA residual voltage = lightning impulse protection level = 806 kV

Peak value of current (A)

Peak

val

ue

of

volt

age

(kV

)



MO Surge Arresters• Main parameters of MO Surge Arresters

– Continuous operating voltage Uc• Maximum rms value of power-frequency voltage, which the arrester can be operated

at, without any type of restrictions. Uc is equal or more than maximal operating voltage in the place where the surge arrester is mounted

– Rated voltage Ur• Maximum rms value of power-frequency voltage under the temporary overvoltage

conditions. It is usually given as amplitude of equivalent overvoltage Ueq with a time period 10 s:

𝑈𝑒𝑞 = 𝑈𝑡𝑇𝑡

10

𝑚,

where Ut is the amplitude of temporary overvoltage, Tt is the overvoltage time duration and m is a constant that depens on surge arrester construction (roughly 0,02)• Usually, the ralatio between Uc and Ur is:

𝑈𝑐

𝑈𝑟=0,8

– Nominal discharge current In• The peak value of lightning current impulse. It is used for surge arrestor classification

when the current impulse 4/10 µs (which the surge arrester must withstand) is assigned to each nominal lightning current impulse 8/20 µs, e.g. : (10 000 A - 100 kA), (5 000 A- 65 kA)



Traveling wave processes• A surge arrester is always placed as close as possible to protected

object to eliminated an influence of reflected voltage waves• The surge arrester protects objects which are inside of so called

protective zone (distance) before and behind of surge arrester – Surge arrester placed in point P has an ignition and

residual voltage Ur

– After the steep voltage wave reaches the interface in point P one part of the wave is reflected and the second part is penetrated through the interface

– The penetrated wave is limited to Ur and the reflected wave has a shape of original wave but with opposite polarity

– The superposition of reflected and original wave causes that the voltage before surge arrestor is lower than would be from comming original wave

– Considering an insulation level of equipment Uithen the protective distance d can be found for given wave steepness S, i.e. the distance in which no voltage higher than Ui will occures



Protective distance of surge arrester• The protective diastance of a surge arrester can be determined from the

similarity of ABC and A´B´C triangles• If z is the distance, which a voltage wave traveled by velocity v in time, in

which the wave reaches Ur after comming to point P, then 𝑧 =𝑈𝑟

𝑆𝑣 and

from the similarity of triangles:𝑈𝑖

2𝑑 + 𝑧=𝑈𝑟𝑧

• The protective distance can be derived as:

𝑑 =𝑈𝑖 − 𝑈𝑟2𝑆

𝑣

z



Protection of electrical systems within structures against lightning

• Lightning protection systems classification– External lightning protection system

• Lightning conductor including connection to internal terminal for equipotential bonding needs

– Internal lightning protection system• Set of measures inside of building to prevent uncontrolled

iniciations of flashovers and breakdowns (including connected electric appliances)

• Equipotential bonding– Is a part of internal lightning protection, which

incorporates direct connection of all conductive parts together and connection of live conductors through surge arresters



External lightning protection system

• Capture device– Meshed cage– Lightning rods (simple or with triggering system)

• Earth down-conductor– Electrical connection of capture device with earthing system

• Earthing system– low voltage drop when the lightning current flows to ground

Meshed cage lightning conductor Air-Termination Lightning Protection System



Internal lightning protection system

• Set of measures to the reduction of electromagnetic impulses effects caused by lighning current inside a protected object:– Equipotential bounding– Usage of surge arresters– Electromagnetic shielding

• It is impossible to reach the effective protection by only one measure -> cascaded or selective protection

• Surge arresters are divided into four classes from A to D Class Way of usage

For overhead line instalation

Lightning current surge arresters for the main potential balancing, zone interface ZBO 0A and ZBO 1

Surge arresters, overvoltage protection in fixed electrical wiring

Surge arresters, overvoltage protection in fixed or flexible electrical wiring




• Selection of surge arrester class is done with respect to lightning protection zones and residual voltage

• Lightning protection zones conception– ZBO 0A – external unprotected area– ZBO 0B – protected by captured devices, direct lightning stroke is impossible– ZBO 1, ZBO 2, ZBO 3 – internal areas

• Surge arrester types– Gas filled spark gaps

• Function depends on steepness of voltage impulse, time response approx. 100 ns

– Varistors• Bipolar elements with voltage dependency (SiC, ZnO), minimal consequent current, time

response approx. 20 ns

– Voltage supressors • Special types of Zener’s diod, TRANSZORB, ZAP, TRANSIL




• Cascading (coordination) of lightning protections from the main distribution power panel

Overvoltage category(IEC 664)Withstand impulse voltage

Class of surge arrester

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